The exemplary embodiment relates to the design, synthesis and devices comprising a class of n-type (electron-mobile) conjugated compounds which can have exceptionally high electron affinities. The exemplary compounds (monomers, oligomers, and polymers) contain highly electron-accepting cyclic diborylene units. The compounds find particular application in electronic devices, such as organic photovoltaic cells (OPVs), and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amendable to other like applications.
Specific functions of many electronic components and devices arise from the unique interactions existing between p-type and n-type conducting and semiconducting materials. Inorganic conductors and semiconductors entirely dominated the electronic industry until a few years ago. Recently, there has been a major worldwide research effort to develop conducting and semiconducting organic compounds and polymers, and to use them to fabricate plastic electronic devices, such as organic photovoltaic devices (OPVs), organic light emitting diodes (OLEDs) and organic field effect transistors (OFETs). Plastic electronic components offer several potential advantages over traditional devices made of inorganic materials since they are flexible and can be manufactured by inexpensive ink-jet printing or roll-to-roll coating technologies.
A contemporary OPV cell contains an electron donor and an electron acceptor in the active layer which spaces an anode and cathode. The interface of the two materials is called a heterojunction, where two intrinsic chemical potentials for electrons exist corresponding to the energy offset between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of the donor and acceptor. Upon light absorption, electrons in the HOMO of the donor and the acceptor are excited into their respective LUMO to form excitons, i.e., Coulombically bound electron-hole pairs. The Coulombic energy, often called exciton binding energy, must be overcome to further separate the electron and the hole. This is achieved by the energy offsets at the heterojunction. If the LUMO offset is larger than the binding energy of the donor exciton, the electrons in the donor phase transfer into the acceptor phase when the excitons diffuse to the heterojunction. Similarly, if the HOMO offset is sufficient to overcome the binding energy of the acceptor exciton, the excitons in the acceptor phase dissociate at the heterojunction. After charge transfer at the heterojunction, the electrons move away into the acceptor phase, driven by the photoinduced electrical potential and the concentration gradient, and the holes move into the donor phase. The anode and the cathode then collect the electrons and holes, respectively, often with the assistance of a bias potential. The overall energy conversion efficiency of the device depends on the efficiencies of the individual events, i.e., light absorption, exciton diffusion to the heterojunction, exciton dissociation at the heterojunction, and charge transport to the electrodes.
An early heterojunction cell adopted a two-layered planar geometry. See, Tang, C. W., Appl. Phys. Lett. 48, 183-185, 1986. The efficiency of the planar device (˜1%) is inherently limited by the exciton diffusion length, which is ˜5-10 nm in organic semiconductors (Nunzi, J. M., C.R. Physique 3, 523-542, 2002), before the hole and the electron recombine. Excitons formed at the location further than 10 nm from the heterojunction do not contribute to the generation of electrical current.
Bulk heterojunction cells were introduced around the mid 1900s. See, Yu, G et al., J. Science 270, 1789-1791, 1995. These included a blend of C60 as the electron acceptor and a poly(p-phenylene vinylene)-type polymer as the donor to form one single active layer. Within the active layer, the two components segregate into continuous network domains and as such result in a large increase in the donor-acceptor interfacial area. The efficiency of such bulk heterojunction device can reach 5%.
Progress has also been made in the development of OPV materials, some of which were originally developed for the FET and LED applications. Regioregular poly(3-alkylthiophene)s (rr-P3ATs) tend to exhibit good optical absorption characteristics and excellent charge mobility compared to the original donor, poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV). A number of donor polymers with the right HOMO-LUMO bandgap have been developed to give absorption maxima overlapping with the maximum of the solar spectrum. (Shaheen, S. E. et al., Synth. Met. 121, 1583-1584, 2001, Brabec, C. J. et al., Adv. Funct. Mater., 12, 709-712, 2002, Smith, A. P. et al., Chem. Mater, 16, 4687-4692, 2004, Muhlbacher, D. et al., Adv. Mater, 18, 2884-2889, 2006). With respect to the acceptors, the soluble C60 derivative [6-6]-phenyl C61-butyric acid methyl ester (PC60BM) is now widely used in OPV devices. The C70 analog (PC70BM) has also been developed to improve the optical absorption coefficient. Currently, the most efficient OPV devices are the rr-P3AT/PCBM or rr-P3AT/PC(70)BM blends.
Electron-donor conjugated polymers are generally easier to design than electron-acceptor conjugated polymers. An electron-donor polymer can be created by appropriate introduction of an electro-negative heteroatom such as sulfur, nitrogen or oxygen into the conjugated polymer. A variety of chemistries are available for introducing electro-negative heteroatoms into such polymers. Therefore, a large number of p-type conducting polymers have been developed and characterized. Additionally, many p-type conducting and semiconducting polymers have been used in commercial devices. They are successfully competing with conventional inorganic semiconductors and conductors.
It is more difficult to design electron-acceptor conjugated polymer systems. Currently, most of the polymers used as n-type semiconductors are hydrocarbon-based polymers such as poly(phenylenevinylene), carrying electron-withdrawing substituents such as cyano or nitro groups polymers containing oxadiazole, quinoxaline, or pyridine units, and a few ladder polymers such as BBL ({poly(7-oxo, 10H-benz[de]imidazo[4′,5:5,6]-benzimidazo[2,1-a]isoquinoline-3,4:10,11-tetrayl)-10-carbonyl}). Unfortunately, current n-type semiconducting polymers have generally poor properties which include low charge carrier density and low carrier mobility. Additionally, most of these materials are difficult to process, and some of them are difficult to synthesize.
In some cases, n-type semiconducting non-polymeric species, such as functionalized fullerenes, molecular glasses and metal complexes, are used instead of polymers (Strohriegl, P. et. al., Advanced Materials, 14, 1439-1451, 2002; Shaheen, S. et. al., Appl. Phys. Lett, 78, 841-843, 2001). The disadvantage of these non-polymeric semiconducting species is the low charge carrier mobility due to the limited conjugation due to low molecular weight and the fact that they often need to be processed by vacuum deposition techniques. Particularly, fullerene derivatives have inherent shortcomings as a critical component in OPVs. A well-recognized problem is the low absorption coefficients of C60 derivatives and to a lesser extent, C70 derivatives in the solar spectrum. The problem itself is usually surmountable by tailoring the structure of the π-conjugated system, but in the case of fullerenes, their closed structures make such chemical modification very difficult.
There are two basic ways to produce a pi-conjugated polymer structure that is an electron acceptor. First, the conjugated backbone of the polymer can be chemically modified by substitution with electron withdrawing substituent groups. Pendant modification effectively imparts some electron affinity to the pi-conjugated polymer. As an example, poly(para-phenylene vinylene) has been modified with cyano and other pendant groups to produce a pi-conjugated semiconducting polymer with n-type properties. Another more effective way to impart n-type semiconducting properties is to directly modify the backbone of the polymer with electron affinity atoms or organic structures. Both the oxadiazole and quinooxaline structures are known to impart electron affinity in molecules. Pi-conjugated oxadiazole-containing polymers have been prepared that exhibited n-type semiconducting properties and photoluminescence. Pi-conjugated quinoxaline-containing polymers have also been prepared that also exhibited n-type semiconducting properties, photoluminescence, and electroluminescence. Pi-conjugated polymers incorporating regioregular dioctylbithiophene and bis(phenylquinoline) units in the backbone of the polymer have additionally been prepared and demonstrated both polymer light-emitting diodes (PLEDs) and organic field-effect transistors (OFETs) prototype devices utilizing these materials.
Due to the valence electronic structure of the boron atom and its ability to form multiple stable bonds with carbon atoms, certain non-polymeric, pi-conjugated, organoboron molecules have been observed to be electron acceptors (Nada, et al., J. Am. Chem. Soc., 120, 9714-9715, 1998; Matsumi, et al., Polymer Bulletin, 50, 259-264, 2003). The empty p-orbital of boron can join in the pi-conjugated system without any added electron density. Mono- and di-vinylhaloboranes and trivinylborane have been extensively studied due to the possibility of delocalization of pi electrons between the vacant p orbital of boron and the pi orbitals of conjugated organic substituents. These molecules exist only in a planar conformation suggesting that there is delocalization of the vinyl pi electrons over the boron atom (Pelter, A., and Smith, K. “Triorganylboranes,” in Comprehensive Organometallic Chemistry, Vol 3, 792-795, 1979). Theoretical calculations performed with the LCAO and self-consistent field methods (Good, C. D., and Ritter, D. M., J. Am. Chem. Soc., 84, 1162-1165, 1962) as well as 13C-NMR studies (Yamamoto, Y. and Moritani I., J. Org. Chem., 40, 3434-3437, 1975) also predict considerable delocalization of the vinyl pi electrons over the carbon-boron bonds. Three-coordinate boron species are reported to be equivalent to carbonium ions and are, therefore, extremely electron-deficient systems. Yet, if the boron is sterically protected with bulky trimethylphenyl groups, as an example, the resultant materials are air-stable (Marder et al., J. Solid State Chemistry, 154, 5-12, 2000). Low molecular weight, non-polymeric, pi-conjugated organoboron compounds are reported to have redox properties that are analogous to nitrogen-containing pi-conjugated molecules. In fact, under chemical or electrochemical reduction, organoboron compounds form a series of anions of the type: —BR2, —BR2.−, ═BR2−, while nitrogen-containing compounds upon oxidation form the series of cations: —NR2, —NR2.+, ═NR2+ (Fiedler et al., Inorg. Chem., 35, 3039-3043, 1996) indicating that pi-conjugated organoboron compounds are redox active and are effectively easy to reduce. The use of certain organoboron, non-polymeric pi-conjugated molecules as an electron transport layer (ETL) in molecular organic light-emitting diodes has also been reported. They report an improvement in maximum luminescence by a factor of 1.6 to 1.8 compared to an identical single layer device that does not contain the organoboron. These organoboron ETL materials are non-polymeric molecules of defined structure having a specific molecular weight and are not pi-conjugated organoboron polymers.
Non-conjugated, organoboron polymers in which sterically bulky organic groups are appended to the boron atoms adjacent to the polymer chain have been reported. A number of pi-conjugated, organoboron polymers that make use of bulky protecting groups have been reported. These polymers have absorption maxima in the visible region and are highly fluorescent when irradiated with UV light which suggests the existence of an extended π-conjugation across the boron atoms. The polymers are also soluble in common organic solvents and stable in air and moisture in the pristine undoped state. Furthermore, the n-doping of a π-conjugated, organoboron polymer with triethylamine has achieved a conductivity of 10−6 S/cm. The n-type semiconducting properties and photoluminescence of these materials have been reported but were not shown to be useful in thin film, organic polymer electronic devices, such as OPVs, PLEDs, or OFETs.
There remains need for n-type semi-conductor materials which are suited to use in electronic devices and other applications where high electron affinities are desired.